[R] Obtaining a quadratic function igven three points on a curve

[Joshua Wiley]

Hi Barth,

Here is an option fitting a linear model toa  second order polynomial
and extracting the coefficients.  The Intercept corresponds to "c" in
your email, then poly(...)1 to "b" and poly(...)2 to "a".

################
dat <- read.table(textConnection("
Y X
0.159529 0
0.5 0.773019
1 1"), header = TRUE)
closeAllConnections()

coef(lm(Y ~ poly(X, 2), data = dat))
#################

For details see:

?poly
?lm
?coef

Hope this helps,

Josh

On Thu, Jan 20, 2011 at 6:42 AM, Barth B. Riley <bbriley at chestnut.org> wrote:
> Hello
>
> I would like to obtain the coefficients for a quadratic function (ax^2 + bx + c) given three sets of points on the quadratic curve. For instance:
>
> Y                       X
> 0.159529                0
> 0.5                     0.773019
> 1                       1
>
> Is there a function in R to obtain the a, b and c ceofficients?
>
> Thanks
>
> Barth
>
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-- 
Joshua Wiley
Ph.D. Student, Health Psychology
University of California, Los Angeles
http://www.joshuawiley.com/